1. Field of the Invention
This invention relates to a water-soluble lubricant for hot plastic working capable of using instead of a graphite- based lubricant in the hot plastic working such as forging, extrusion, drawing or the like.
2. Description of the Related Art
Up to the present, the oily or water-soluble graphite-based lubricant has been used in hot plastic working. The former lubricant is formed by adding an extreme pressure additive or a flux to a mineral oil and dispersing graphite thereinto, so that there is a fear of fuming or igniting through the oil in the hot plastic working, which causes problems in view of the operation environment or health. The latter lubricant is formed by dispersing graphite into water, so that there is no fear of fuming or igniting and the lubricity is good, but there is a problem of blackly contaminating the operation environment with graphite.
In order to solve the problems on the operation environment due to the oily and water-soluble graphite-based lubricants, it has been attempted to develop lubricants for hot plastic working using no graphite. For instance, there are a lubricant using an alkali metal salt of fumaric acid (Japanese Patent laid open No. 58-52395), a lubricant using an alkali metal salt of phthalic acid (Japanese Patent laid open No. 58-84898) and the like. In these lubricants, the problem with respect to the operation environment is solved because graphite is not used, but the lubricity is rather poor as compared with the graphite-based lubricant, so that seizuring is caused or defects are generated in products. Further, if a mold temperature is lower than that in the graphite-based lubricant, the formation of a lubricating film is considerably poor, so that sticking on the product, defects in the product and the like are caused and hence the product can not be put into use.
The lubricants using no graphite are poor in lubricity as compared with the graphite-based lubricant as mentioned above, so that it is necessary to more uniformly form a lubricating film having a given thickness in a mold as compared with the use of the graphite-based lubricant. In a lubricant using the alkali metal salt of fumaric acid or phthalic acid, however, a good lubricating film is formed in a mold at a temperature of 200.degree.-300.degree. C., but when the mold temperature exceeds 300.degree. C., the adhesion to the mold is degraded due to the decomposition of organic adhesive and a good lubricating film is not obtained, and also when the mold temperature is lower than 200.degree. C., an undesirably thinner film is only formed in the mold.